A conventional photovoltaic cell incorporates a semiconductor structure with a junction, such as a p-n junction formed with an n-type semiconductor and a p-type semiconductor. For the typical p-base configuration, a negative electrode is located on the side of the cell that is to be exposed to a light source (the “front” side, which in the case of a solar cell is the side exposed to sunlight), and a positive electrode is located on the other side of the cell (the “back” side). This cell has a front n-type emitter silicon surface. Radiation of an appropriate wavelength, such as sunlight, falling on the p-n junction serves as a source of external energy that generates electron-hole pair charge carriers. These electron-hole pair charge carriers migrate in the electric field generated by the p-n junction and are collected by electrodes on respective surfaces of the semiconductor. The cell is thus adapted to supply electric current to an electrical load connected to the electrodes, thereby providing electrical energy converted from the incoming solar energy that can do useful work. Solar-powered photovoltaic systems are considered to be environmentally beneficial in that they reduce the need for fossil fuels used in conventional electric power plants.
Industrial photovoltaic cells are commonly provided in the form of a structure, such as one based on a doped crystalline silicon wafer, that has been metalized, i.e., provided with electrodes in the form of electrically conductive metal contacts through which the generated current can flow to an external electrical circuit load. Most commonly, these electrodes are provided on opposite sides of a generally planar cell structure. Conventionally, they are produced by applying suitable conductive metal pastes or inks to the respective surfaces of the semiconductor body and thereafter firing the pastes.
Conductive pastes are typically used to form the conductive grids or metal contacts. Conductive pastes typically comprise a conductive species (e.g., silver particles), a glass frit, and an organic medium. To form the metal contacts, conductive pastes are printed onto a substrate as grid lines or other patterns and then fired, during which electrical contact is made between the grid lines and the semiconductor substrate.
However, crystalline silicon solar cells are typically coated with an anti-reflective coating (ARC) such as one or more of silicon nitride, titanium oxide, or silicon oxide to promote light adsorption, which increases the cells' efficiency. Such anti-reflective coatings also act as an insulator which impairs the flow of electrons from the substrate to the metal contacts. To overcome this problem, the conductive ink should penetrate the anti-reflective coating during firing to form metal contacts having electrical contact with the semiconductor substrate. This type of process is generally called “fire through” or “etching” of the insulating ARC. Formation of a strong bond between the metal contact and the substrate and solderability is also desirable.
The ability to penetrate the anti-reflective coating and form a strong bond with the substrate upon firing is highly dependent on the composition of the conductive ink and firing conditions. Efficiency, a key measure of solar cell performance, is also influenced by the quality of the electrical contact made between the fired conductive ink and the substrate.
Alternatively, a reverse solar cell structure with an n-type silicon base is also known. This cell has a front p-type silicon surface (front p-type emitter) with a positive electrode on the front-side and a negative electrode to contact the back-side of the cell. Solar cells with n-type silicon bases (n-type silicon solar cells) can in theory produce higher efficiency gains compared to solar cells with p-type silicon bases owing to the reduced recombination velocity of electrons in the n-doped silicon.
To provide an economical process for manufacturing solar cells with good efficiency, there is a need for thick-film paste compositions that can be fired at low temperatures to penetrate an anti-reflective coating and provide good electrical contact with the semiconductor substrate.